Intermittent bank 1 misfire with significant fuel trim swings - need help

I have a 2002 C4S. I have a misfire that I have been dealing with for the last 6 months and I just can't figure it out. Basically I get misfires on bank 1 (P0300, P0301, 302 and 303) codes. This occurs intermittently, you can go a few hundred miles between individual occurrences. What is interesting is that when it does happen, it is usually after being on a highway at constant high speed for a while (engine RPM around 3k) and then coming to an exit. Looking at the freeze frame data, the misfire happens at around 1000 rpm, the long term fuel trim on bank 1 is 2.3% but a whopping -22.7% short term. On bank 2 fuel trims are 1.6% long term and 21.9% short term. So clearly something is driving computer to take away a lot of fuel on bank 1 while at the same time add a lot of fuel to bank 2. Also, the misfire is very brief, you can feel a bit of engine vibration for may be a second or two and then it goes back to normal. Again, this is very intermittent and I can't get it to replicate on demand. I have driven around with a handheld OBD scanner which shows all 4 fuel trims, and most of the time they are all within a normal range (plus/minus 10%) and the engine runs perfectly fine. It is just intermittently something causes it to swing lean on one bank and rich on another at the same time. Anyone have any ideas of what this can be?

Looking at earlier posts you stated having replaced the short block and having head work done. Did you replace any of the valve train and or associated parts?

The usual suspects r o2 and maf sensors. Best thing to do is graph them during the event. Worst thing to do is to start throwing parts at it. Although most do the later due to the costs of diagnostic tools.

Jumpy chunky, happy to provide more details. I had a valve seat drop on cylinder 3 in 2018. It destroyed the head and scored the bore in cylinder 3. I considered my options and decided to buy a shortblock from Porsche and a used cylinder head which was sent to machine shop prior to installation. Cylinder head from bank 2 was also sent to machine shop, but I reused my old one. This essentially new engine was put into the car in 2018 and it went for almost 20k miles without any issues. Earlier this year I started having intermittent misfires. Using a basic OBD tool I would get codes p0300 and P0303. This was weird as I couldn't understand why I get a p0300 (multiple cylinder misfires) if the misfire is only on a single cylinder. I tried the usual, like spark plug and coil on cylinder #3 but that didn't fix it. Again, the misfire was intermittent, can go several hundred miles before it occurred. And when it did occur it was brief. Then I decided that perhaps it is an intake lifter on cylinder 3. There have been many examples here of a stuck intake lifter causing intermittent misfires. I ended up replacing all 6 intake lifters on bank 1 of the engine. That's about $1000 in parts alone. I did it with the engine in the car. Not the easiest job, but certainly doable without taking the engine out. To my disappointment this didn't solve the problem. However, I did find that when a regular OBD2 scanner shows code p0300 and p0303, using durametric I actually also have misfires for cylinder 1 and 2. For some reason they don't come up on a regular OBD tool. Anyhow, I now think that all along the computer was noticing misfires on cylinders 1,2, and 3, but I assumed that it was only 3 because that is what I saw with my OBD scanner. Anyhow, I am now in a situation where I get this intermittent misfire on bank 1. I knwo that I set timing correctly because my deviations are <4%, and if the timing would be off then I would get misfires very quickly. I wouldn't have to drive 2 weeks and 250 miles just to have one misfire occurrence. I also see that what happens is a large fuel trim deviation on both banks when the misfire occurs. I can't see how that can be an oxygen sensor or a MAF because the misfires are very intermittent, I can go 200-250 miles without seeing it. I looked at my Oxygen sensor voltage graphs and they all seem fine. The intermittent nature of this issue, and my inability to replicate it on demand, is what makes this a tough problem to figure out. This is why I would be curious if anyone can come up with somekind of a fault that will lead one bank to go very lean and another bank rich at the same time, but then run perfectly fine most of the time.

The only thing I have seen cause the fuel trims to go full rich one one bank and full lean on the other is if the 02 Sensors is switched ( yes it is physically possible) ... It's hard to see how it can stay synchronized for 200-250 miles though... but I suppose in some cases it could take that long for the DME to go apesh*t .. I would verify 02 sensor position by watching the 02 Sensor function on Durametric then un-plugging one-by-one and seeing which one stops working... simple...

O2 sensors switched around (front to rear) is a possibility but it does not explain why it ran for 20,000 miles before the problem shows up. Nevertheless, that is easy to check. How old are your knock sensors? Did you re-use the old ones when the engine is redone?

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I disregarded the previous 20k miles ( although any background is helpfull) because literally anything can happen in 20k miles. I have been in Dealerships since 1974 and have seen numerous issues straight off the factory delivery truck to high mileage cars..So I take everything with a grain of salt unless it has a direct bearing on symptoms or test results.. The only possibility that I know of that can cause this issue is the 02 sensors are swapped... The fuel trim system is treated as two seperate 3 cylinder engines, with the same fuel pressure and throttle body...and they have separate "feedback" from their 02 sensors.. My theory is that the exhaust system was recently removed and re-installed and the OP left out that detail because there is no fault codes for 02 sensors and he has seen that the 02 sensors seem to be functioning fine and he wanted to stay concise and focus on the fault codes.... If the two 3 cylinder engines are in perfect tune ( ie valve seal, compression , clean fuel injectors ect) there is no need for the fuel to be trimmed and can run that way for a long time and explain the 200-250 miles before it goes haywire...If one bank has some iffy fuel injectors or valve seal or compression, the fuel trim would go haywire very quickly... Since the two 3 cylinder engine cannot be "absolutely perfect" once the DME starts to make a small correction, the feedback from the appropriate 02 sensor does not happen,( infact the small correction feeds back to the other "engine" ..) then the DME tries to make another small correction, this cycle happens to both engines and keeps going in the wrong direction... If this had been an earlier DME it would have set a fuel trim fault code before it got so out-of-whack that it caused a misfire...Earlier DME's had a 10% window of adjustment for fuel trim and would have been reached and set a fuel trim fault code whereas the later DME's have a 25% window of adjustment for fuel trim...The misfires start to occur at 20%-24% and is just before reaching the limit of the fuel trim to set a fault code for that model... I usually don't comment with "theories"especially if all the relevant info isn't given and there are a lot of relevant info that could have been given such as fuel pressure, full actual values, compression test ,blow-by test , graphs, but it is more info than is usually given and is enough for me to form a hypothesis..

Thanks everyone for the feedback regarding O2 sensors. They are definitely not swapped around. I don't think that you can actually physically connect O2 sensors from bank 1 to bank 2 or vise versa because the wires would not be long enough. Also, even if they were swapped, this would not explain the issue because the engine recovers on its own. If they were swapped and it would drift far off center, I think that you would need to restart the engine to get it to "re-zero". However, I would be driving, coming off the highway and get a check engine light. The engine will hesitate a bit and have a lumpy idle but will continue running and I drive home without a problem. Again, it can take a week of daily driving to to get this to re-occur. I also looked at fuel trims once I get home from a drive where the check engine light was set and the trims are fine. Another words once the misfire event passes the fuel trims get back to normal which is why the engine runs fine. They only swing wildly during the brief misfire, and I know their exact value during the misfire because it is saved in the freeze frame, but once the event is over everything goes back to normal operation.

Yes it is physically possible ,the bank 1 pre-cat 02 sensor is literally within inches of the bank 2 post -cat 02 sensor. I have diagnosed this issue several times although the car will usually run terrible and will not "correct it's self" unless a fault code is erased..